关联映射

关联映射之一对一

数据准备

create table teacher(
t_id int primary key auto_increment,
t_name varchar(20)
);
insert into teacher(t_name) values('ZhangSan');

对应的model

@Data
public class Classes {
    private Integer id;
    private String  name;
    private Teacher teacher;
}

班级

create table classes(
c_id int primary key auto_increment,
c_name varchar(20),
t_id int 
);
insert into classes(c_name,t_id) values('Class One',1);

对应的model

@Data
public class Teacher {
    private Integer id;
    private String name;
}

mapper.xml 文件,使用了 元素映射一对一的关联关系。

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.chinaunicom.kg.mapper.ClassesMapper">
    <select id="selectClassById" parameterType="int" resultMap="resultClassMap">
        SELECT c.c_id,
               c.c_name,
               t.t_id,
               t.t_name
        FROM classes c
                 LEFT JOIN teacher t ON c.t_id = t.t_id
        WHERE c.c_id = #{id}
    </select>
    <!-- resultMap: 映射实体类和字段之间的一一对应的关系 -->
    <resultMap id="resultClassMap" type="com.chinaunicom.kg.domain.entity.Classes">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <!-- 一对一关联映射：association -->
        <association property="teacher" javaType="com.chinaunicom.kg.domain.entity.Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
    </resultMap>
</mapper>

关联映射之一对多

学生表

create table student(
s_id int primary key auto_increment,
s_name varchar(20),
c_id int
);
insert into student(s_name,c_id) values('s1',1);
insert into student(s_name,c_id) values('s2',1);
insert into student(s_name,c_id) values('s3',1);

对应的实体student

@Data
public class Student {
    private Integer id;
    private String name;
}

对应的classes修改成

@Data
public class Classes {
    private Integer id;
    private String  name;
    private List<Student> students;
}

对应的mapper.xml,使用了 元素映射一对多的关联关系。

 <!-- resultMap: 映射实体类和字段之间的一一对应的关系 -->
    <resultMap id="baseClassMap" type="com.chinaunicom.kg.domain.entity.Classes">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <!-- 一对多关联映射：collection -->
        <collection property="students" ofType="com.chinaunicom.kg.domain.entity.Student">
            <id property="id" column="s_id"/>
            <result property="name" column="s_name"/>
        </collection>
    </resultMap>
    <select id="selectClassAndStudentsById" parameterType="int" resultMap="baseClassMap">
        SELECT c.c_id, c.c_name, s.s_id, s.s_name
        FROM classes c
                 LEFT JOIN student s ON c.c_id = s.c_id
        WHERE c.c_id = #{id}
    </select>